1. Field of the Invention
The present invention relates to an optical device that is an optical sensor for sensing a parameter such as strain or temperature; and more particularly to a Fabry-Perot sensor having optical fibers embedded into an all-glass structure, the glass structure in some applications having strain amplification properties.
2. Description of the Related Art
Fiber Fabry-Perot (FFP) sensors have been used extensively in the field of fiber optic sensing. The FFP sensor has a particular advantage of being very simple to construct, particularly using a format known as the Extrinsic Fabry-Perot Interferometer (EFPI). Such a device is formed by taking two fiber ends and inserting them into an alignment tube with an inner diameter slightly larger than that of the outer diameter of the fibers. The fibers are set in the tube so as to have a predetermined gap between their end faces. Such glass-collapsing technology is shown and described, for example, in U.S. patent application Ser. No. 09/455,867 (CiDRA File No. CC-0036B), filed Dec. 6, 1999, now U.S. Pat. No. 6,422,084 issued Jul. 23, 2002, as well as U.S. patent application Ser. No. 09/455,865 (CIDRA File No. CC-0078B), filed Dec. 6, 1999, now U.S. Pat. No. 6,519,388 issued Feb. 11, 2003. A device so formed can be used as a Fabry-Perot sensor, which can be interrogated along either fiber lead. With uncoated fiber ends, the so-called finess of the FFP is very low (nominally 2), but can be increased by coating the fiber end faces with suitable coatings. To use an EFPI as a pressure sensor element, an arrangement is provided by which the optical path difference (OPD) between the two end faces depends on the pressure being sensed, for example through a mechanical coupling of one fiber to a pressure responsive diaphragm.
In providing an FP-element-based sensor, it is advantageous to embed an FP element (i.e. an FFP etalon with a cavity having a variable optical path length) in a structure that will respond suitably to pressure. It is known in the art to collapse a glass tube onto two fibers. For example, such glass collapsing technology is shown and described in U.S. patent application Ser. No. 09/455,867 (CiDRA File No. CC-0036B), entitled Bragg Grating Pressure Sensor, filed Dec. 6, 1999, now U.S. Pat. No. 6,422,084 issued Jul. 23, 2002, as well as U.S. patent application Ser. No. 09/455,865 (CiDRA File No. CC-0078B), entitled Tube-encased Fiber Grating, filed Dec. 6, 1999, now U.S. Pat. No. 6,519,388 issued Feb. 11, 2003, both hereby incorporated by reference in their entirety. It is also known to use the resulting glass element from the glass collapsing technology as a transducer. The glass collapsing technique produces high stability grating-based sensors that provide high repeatability and low hysteresis.
What is needed is an FP element making use of structures provided by the aforementioned glass collapsing technique, FP elements that can be used as sensors.